This invention relates to a device for draining aqueous humor from a human eye for the relief of glaucoma.
Glaucoma is an eye condition in which, from various causes, the intra-ocular pressure (that is to say the pressure of the aqueous humor in the eye) rises, tending to make the eyeball hard and, in high-tension glaucoma, adversely affecting vision and even causing partial or total loss of sight.
A known method of treating glaucoma is filtration surgery, either with or without implanted drainage devices. In a filtering operation which does not involve an implanted drainage device, an opening is cut into the anterior chamber of the eye (the opening being formed under a flap of conjuctiva) to drain aqueous humor to a bleb which forms over the sclera. The aqueous humor in the bleb seeps away over a period to small blood vessels and lymphatic vessels. Filtering operations of this type are reasonably successful. Unfortunately, however, about 20% fail over a period of time following the operation, and about 30% lead to cataract formation. The main reason for these problems is a result of poor control of the filtering opening, and subsequent reduction in the drainage flow of aqueous humor. The resulting ocular hypotony may lead to cataract formation, and does lead to the apposition of tissues and scarring at the site of the sclerostomy, and this gives rise to failure of the operation.
Although various attempts have been made to overcome these problems by the use of implant devices, none of these has proved entirely successful. Implant devices utilise drainage tubes to maintain the integrity of the openings formed in diseased eyes for the flow of aqueous humor. Typically, the drainage tube of such a device leads to a drainage body which is sutured to the sclera. The purpose of the drainage body is to increase the drainage area, and hence to ensure that aqueous humor drains away at a sufficiently high rate. After such a device is surgically implanted, scar tissue forms around the drainage body, the aqueous humor filtering through this scar tissue to the blood and lymphatic vessels. This scar tissue helps to immobilise the device. However, if the scar is too thick, it resists filtration of aqueous humor to the surrounding blood and lymphatic vessels. In this connection, a thick scar is analagous to multiple layers of filter paper, in that both require a larger surface area (than a thin scar or a single layer of filter paper) to ensure the same rate of filtration. Thus, when thick scar tissue forms with this type of device, failure follows owing to inadequate surface area of the drainage body. In a normal eye, the pressure of aqueous humor in the anterior chamber is about 14 to 16 mm of mercury. A device may be said to have failed, when the pressure in the anterior chamber is greater than 18 mm of mercury. A medically accepted test for a successful drainage device is one that ensures that the intra-ocular pressure remains below 18 mm of mercury for at least six months following implantation.
A typical known implant device (see British Patent Specification No. 2 101 891) utilises a thin silicone rubber drainage tube which leads to one or more ridged circular plates. The circular plate(s) form a drainage body, which is sutured to the sclera of the eye. This type of device has a high failure rate, because the surface area of the drainage body is inadequate to ensure sufficient drainage of aqueous humor.
The object of the invention is to provide an aqueous humor drainage device which does not suffer from the disadvantages of known devices, and which can ensure that the intra-ocular pressure remains below 18 mm of mercury for at least six months following implantation for the majority of patients.